Understanding A Building’s Thermal Performance Using U-values

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The U-value of a building element, such as a wall, roof or floor, is a measure of the thermal transmittance of that element. The lower the U-value, the slower the flow of heat is through that element once it has reached a steady state (meaning the flow of heat through is becomes constant). In the UK, Building Control implement Part L of the building regulations which require you to achieve a minimum thermal performance. This changes from time to time so it is best to check with your Building Control officer what you need to achieve.

To calculate U-values it is easier to turn them into an R-value (thermal resistance, the inverse of U-value or 1/U) then you can simply add the resistance of each layer to calculate the total thermal resistance of the wall. This method is used to calculate simple U-values but as wall structures become more complex, such as with the additional of metal or timber studs, joists, roof structures, etc., the calculation becomes more complex and is best left to software to calculate.

Once you know the overall U-value of an element it is quite simple to calculate an approximate heat loss at a given temperature which helps you size your heating system. Simply multiply the U-value (in W/m2K) by the temperature difference between inside and outside (e.g. 20C inside and -1C outside is a 21C difference) and then multiply by the surface area in m2. E.g. a 100m2 wall with a U-value of 0.2 W/m2K and a temperature difference of 21C will need at least 0.2 x 100 x 21 W or 420W to keep the inside at 20C.

To calculate the total heat loss of the house though the building fabric you’ll need to do the same for the roof, and floor and also the doors and windows. You also lose heat from ventilation/draughts though so don’t forget to allow for 0.36W of heat to heat 1m3 of air through 1 degree and assume that you change the entire volume of air in the house between 1-2 times per hour.

Whilst it may seem easy to compare the performance materials by comparing U-values it is very important to remember that U-value is not the only factor that affects thermal performance. Specific heat capacity, density and diffusivity all play a large role in how a material performs and consequently how much energy a building uses.

Highly insulating, lightweight materials, such as PIR boards or high performance glass wools, often cause buildings to use more heating than dense, slightly less insulating materials such as wood fibre insulation. Some research shows that heavyweight buildings with the same overall U-value as lightweight buildings require 25% less heating to stay at the same temperature. There are various reasons for this including time taken to reach a steady flow of heat through the construction and also heat stored by the material itself.

If you have a building project and you’d like us to calculate U-values and suggest insulation materials to get the best performance from your building, please contact us.

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